The secret of a geothermal system's clean, efficient operation is the earth's ability to store heat. 47 percent of the sun's energy that reaches our planet is absorbed into the earth. That's 500 times more energy than all of humanity would need in a year! It's an unlimited source of energy.

Installing a geothermal system in a typical home is the environmental equivalent of planting 750 trees -- or taking two cars off the road. That's because geothermal systems don't burn fossil fuels, and they don't emit carbon dioxide (which has been associated with the greenhouse effect and global warming).

The Geothermal Heat Pump Consortium says current geothermal systems save more than 14 million barrels of crude oil every year. And because the only energy needed to run geothermal systems is a small amount of electricity, they reduce the need for new coal-powered electric power plants. Result? Cleaner air, less acid rain -- and MUCH lower energy bills. (I have a friend who installed a geothermal system in his house 14 years ago. It's about a mile from our new house and a bit larger than ours. His ANNUAL electric bill (heat, AC, light, cooking, hot water, etc.) has averaged a little over $600.

A geothermal exchange heat pump, also known as a ground source heat pump, is a heat pump that uses the Earth as either a heat source, when operating in heating mode, or a heat sink when operating in cooling mode.

All geothermal heat pumps are characterised by an external loop containing water or a water/antifreeze mixture (propylene glycol, denatured alcohol or methanol), and a much smaller internal loop containing a refrigerant. Both loops pass through the heat exchanger. Air source heat pumps use the same principle but extract the heat from the air, rather than the ground. As such their installation is much simpler and cheaper.

There are three categories of geothermal heat pumps based on the type of external loop: open loop system, closed loop vertical system, and closed loop horizontal system.

In an open loop system the thermal transfer fluid (water) does not return and is a 'once through' type system. The open loop system draws water from a well or lake, passes it through a heat exchanger in the building, and then discharges it. The water can be discharged to a stream or lake, or injected into a second well.

The pipe in vertical closed loop system uses a single well (or borehole) with the fluid in the pipe constantly recirculated to and from the well. If a borehole is used, it is commonly filled with a bentonite grout surrounding the pipe to provide a good thermal connection to the surrounding soil or rock.

The horizontal closed loop is placed below the frostline (1 to 2 meters) underground. In a horizontal closed loop system the pipe is often laid out as a spiral, colloquially known as a "slinky", to increase the contact area per length. A typical traditional 2,000 sq. ft. (185.8 m ) residence will require four tons oil equivalent (14.06 kilowatts) of heating capacity, while a well insulated modern residence of equal size should typically consume no more than 1 ton.

The amount of vertical or horizontal loop required for each ton of capacity is a function of the soil formation thermal conductivity and deep earth temperature, and also depends on the balance between the amount of heat rejected to and absorbed from the ground during the course of the year. A rough approximation of the soil temperature is the average daily temperature for the region. The heat in the soil is from the sun.

Geothermal heat pumps are also used in non-residential buildings, but the variety of loads and load patterns in these applications make it difficult to specify rules of thumb for capacity per unit of building area, or quantity of heat exchanger required per unit of heat pump capacity. In commercial applications a field of bore holes is drilled. Bore holes are spaced 15-20 feet apart and generally at 150-200 feet deep per ton of cooling. During the cooling season, the local temperature rise in the bore field is influenced most by the moisture travel in the soil. Reliable heat transfer models have been developed through sample bore holes as well as other tests.

Heat pumps are especially well matched to underfloor heating systems, rather than wall mounted radiators, and so are ideal for use in open plan offices. Using large surfaces such as floors, as apposed to radiators, distributes the heat more uniformly and allows for a lower temperature heat transfer fluid.

The Earth below the frost line remains at a relatively constant temperature year round, usually between 7-21 degrees Celsius (45-70 degrees Fahrenheit) depending on geographical location. Becauses this temperature remains constant, geothermal heat pumps perform with far greater efficiency and in a far larger range of extreme temperatures than conventional air conditioners and furnaces.